Started on grids in UI. Initial work - brainstorming, outline, etc.
add support for "type", "col", "row" attributes in `drag_and_drop_input`
Initial front end logic.
add `clean_user_answer`, add test for type="grid"
Updated comments.
Changed naming scheme for grid cells.
add get_all_items, Draggable, DraggableSet
Fix for gighest target z-index.
Fixed IE10 bug. Now can drag without starnge behavior across all browsers.
refactoring: `on_target()` -> `on()`
Cherry picked commit abc8555f65d6f89d72761220c55eef63df74ae51.
Documenting ^_^v.
chem mode in DnD: separate labels in slider and icons in slider of same height are fixed
mathjax for label end: Mathjax labels are added
add "auto_resize" and "separate_labels" options for DnD, make lines in DnD prettier
chem DnD: Chem flags were partly implemented
Support new attributes for chemical problems. JS part is implemented
add DnD constraints documentation
use lower case for bool value in DragAndDropInput attrs
pep8 formating